Development of dynamic spectrum access and allocation techniques recently have made feasible the vision of cognitive radio systems. However, a fundamental question arises: Why would licensed primary users of a spectrum band allow secondary users to share the band and degrade performance for them ? This incentive issue has been sought to be addressed by designing incentive-compatible auction mechanisms. This, however, does not solve the problem as the auctioneer (usually the primary user) is also an interested party. So, why would the secondary user trust the primary user to not manipulate the auction. We propose that a more appropriate mechanism to solve this incentive problem is a contractual mechanism. In this paper, we consider a simple setting: A single primary transmitter-receiver pair and a single secondary transmitter-receiver pair with a Gaussian interference channel between them. We consider the setting of complete information when channel attenuation coefficients and noise levels at the receivers are common knowledge. We consider when receivers cooperate to do successive-interference cancellation. In contrast to the results of for unlicensed bands, we show that it is possible to achieve socially optimal rate allocations with contract mechanisms in licensed bands.